The present invention relates to water transport plates. More specifically, the present invention relates to water transport plates and a method of use thereof.
Solid polymer electrolyte fuel cell power plants are known in the prior art, and prototypes are even available from commercial sources, such as Ballard Power Systems, Inc. of Vancouver, Canada. These systems are serviceable, but are relatively complex. An example of a Ballard Power Systems polymer membrane power plant is shown in U.S. Pat. No. 5,360,679, granted Nov. 1, 1994. One problem occurring in solid polymer fuel cells relates to the management of water, both coolant and product water, within the cells in the power plant. In a solid polymer membrane fuel cell power plant, product water is formed by an electro-chemical reaction on the cathode side of the cells, specifically by the combination there of hydrogen ions, electrons and oxygen molecules. The product water must be drawn away from the cathode side of the cells, and makeup water must be provided to the anode side of the cells in amounts which will prevent dryout of the proton exchange membrane, while avoiding flooding, of the cathode side of the electrolyte membrane.
Austrian Patent No. 389,020 describes a hydrogen ion-exchange membrane fuel cell stack which utilizes a fine pore water coolant plate assemblage to provide a passive coolant and water management control. The Austrian system utilizes a water-saturated fine pore plate assemblage between the cathode side of one cell and the anode side of the adjacent cell to both cool the cells and to prevent reactant crossover between adjacent cells. The fine pore plate assemblage is also used to move product water away from the cathode side of the ion-exchange membrane and into the coolant water stream; and to move coolant water toward the anode side of the ion-exchange membrane to prevent anode dryout. The preferred directional movement of the product and coolant water is accomplished by forming the water coolant plate assemblage in two parts, one part having a pore size which will ensure that product water formed on the cathode side will be wicked into the fine pore plate and moved by capillarity toward the water coolant passage network which is inside of the coolant plate assemblage. The coolant plate assemblage also includes a second plate which has a finer pore structure than the first plate, and which is operable to wick water out of the water coolant passages and move that water toward the anode by capillarity. The fine pore and finer pore plates in each assemblage are grooved to form the coolant passage network, and are disposed in face-to-face alignment between adjacent cells. The finer pore plate is thinner than the fine pore plate so as to position the water coolant passages in closer proximity with the anodes than with the cathodes. The aforesaid solution to water management and cell cooling in ion-exchange membrane fuel cell power plants is difficult to achieve due to the quality control requirements of the fine and finer pore plates, and is also expensive because the plate components are not uniformly produced.
In the fuel cell technology, the water transport plate is a porous structure filled with water. During fuel cell operation, the water transport plate supplies water locally to maintain humidification of a proton exchange membrane (PEM), removes product water formed at the cathode, removes by-product heat via a circulating coolant water stream, conducts electricity from cell to cell, provides a gas separator between adjacent cells and provides passages for conducting the reactants through the cell. The water transport plate supplies water to the fuel cell to replenish water which has been lost by evaporation therefrom. Due to the constraints of the water transport plate formation process, these plates are costly to manufacture and possess limited strength.
For example, water transport plates can be formed in a dry-laid process where graphite powder and powdered phenolic resin are showered into a mold to form a layer. The layer is compacted to form a 0.100 inch thick layer which is heated until the phenolic resin melts and coats the graphite powder. The resin is then cured, thereby bonding the graphite powder in a composite. Although this is a common water transport plate formation process, the forming speed is slow and it is difficult to incorporate relatively long fibers which are desirable for water transport plate structural integrity. Longer fibers tend to become entangled in the dry-laid feeder, thereby forming fiber bundles in the finished composite. This fiber bundling, which corresponds to uneven fiber distribution, creates weak areas within the composite which are susceptible to structural failure. Composite structural integrity is maximized at fiber lengths greater than about 1.0 mm (about 0.040 inches) while the dry-laid process is limited to fiber lengths of about 0.51 mm (about 0.02 inches). Consequently, the tolerances in the specification for the water transport plate are small and the fabrication is difficult, resulting in many rejected parts.
In addition, the environmental and operational parameters of a water transport plate must be carefully balanced to obtain optimum performance of the overall fuel cell. For example, parameters of the water transport plate such as pore size, resistivity, particle size, resin content and yield strength, must be properly selected to obtain bubble pressure characteristics and water permeability which are acceptable for efficient operation of the fuel cell.
In view of the foregoing, an improved water transport plate is desired which has the competing characteristics of bubble pressure and water permeability optimally and properly balanced for efficient fuel cell operation.